Video noise filters have become important differentiators for consumer video products because noise level is one of the key factors of video quality. Videos captured by imaging sensors (e.g., cell phone cameras, video cameras) or received from analog video inputs (e.g., TV cable, DVD/VCR) are often contaminated by significant amounts of random noise. Video noise filters can improve visual quality, coding efficiency, and facilitate further processing and analysis such as enhancement, object tracking, and recognition. Further, video noise filters can be applied in many applications such as cell phone camera, video surveillance, and DVR, etc.
In many applications, a video noise filter includes both a spatial noise filter (SNF) and a temporal noise filter (TNF). In general, temporal noise filtering is more effective than spatial noise filtering for video noise filtering because temporal noise filtering utilizes information from multiple frames. One of the more commonly used TNFs is a temporal infinite impulse response (IIR) filter which has very low computational complexity and utilizes information from all previous frames. However, use of a temporal IIR noise filter on a video sequence may result in ghosting artifacts leading to sticky contours in moving areas of the video sequence, especially at high filter strengths. A sticky contour occurs if an object in a video sequence is moving and the contours, i.e., edges, of the object in a former position remain visible when the object has moved. For example, if a box is moving from left to right, the sticky contours may be visible as a lot of vertical lines to the left of the moving box. These visible contours may be referred to as ghosting artifacts. A motion adaptive temporal IIR noise filter can reduce the ghosting artifacts by automatically adjusting filter strength according to the motion amplitude of the video sequence. However, even with such automatic adjustment, some ghosting artifacts do not fade away with time. Accordingly, improvements in temporal noise filtering are desirable.